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You see the shape is ellipsoid. While it can cover the front side and the back side of the character. It mistakenly includes the vertices above the pants around the lower-part of the t-shirt area. Any ways I can flatten the upper part of the ellipsoid?

You can manually select which vertices you want to be affected by a bone (this means vertices that fall inside or outside of the envelope).

Turn on Edit Envelopes. Bellow the button there's this group called Select. Check the Vertices checkbox.

Now go and manually select all the vertices you want to change influence (use CTRL to add to your selection etc.), go below in the modifier panel until you find the Abs. Effect spinner, in the group Weight Properties.

Move the value down to 0.0 (zero), and the vertices won't be influenced by this bone anymore. Alternatively, with all the vertices selected, you can click the Exclude Selected Verts button (), which tags these verts as "not influenced" by the bone you are editing.